1. Field of the Invention
The present invention relates to a phosphor which emit a light by being excited by light, electromagnetic waves such as X-rays, electron beam, and specifically, relates to a light-emitting device for usual illuminations such as a fluorescent lamp, illuminations mounted on a car, back lights for liquid crystal, displays and the like. Specifically, the present invention relates to a white color and multi-color light-emitting device using a semiconductor light-emitting element.
2. Description of the Related Art
A light-emitting device using light-emitting elements is a small size and superior in electric power efficiency, and emits fresh color. Further, said light-emitting elements have characteristics that there is no fear of a burnt-out light bulb because of a semiconductor element and they are superior in initial drive property and resistant in vibration and the repetition of on-off lighting. Since the light-emitting elements have such superior characteristics, a light-emitting device using semiconductor light-emitting elements such as an LED and a LD has been utilized as various light sources.
There is developed a light-emitting device which emits a luminescence color different from the light of the light-emitting elements by partially or wholly converting the wavelength of the light of the light-emitting elements and mixing said wavelength-converted light with the light of light-emitting elements not subjected to wavelength conversion to release light.
Among these light-emitting devices, a white color light-emitting device has been required in wide fields such as usual illuminations such as a phosphorescent lamp, illuminations mounted on a car, displays and back lights for liquid crystal. Further, there is required a light-emitting device having various color tastes such as a pastel color by combining a semiconductor light-emitting element and a phosphor.
The luminescence color of a light-emitting device using a white color semiconductor light-emitting element is obtained by the theory of color mixture. Blue light released from a light-emitting element is irradiated in a phosphor layer, then repeats absorption and scattering several times in the layer, and then, is released to outside. On the other hand, the blue light absorbed in the phosphor works as an excitation light source and emits yellow fluorescent light. The mixture of the yellow light and the blue light is visualized as white to human eyes.
For example, a blue color light-emitting element is used as the light-emitting element, and a phosphor is thinly coated on the surface of said blue color light-emitting element. Said light-emitting element is a blue color light-emitting device using an InGaN-base material. Further, the phosphor uses a YAG-base phosphor represented by the composition formula of (Y, Gd)3(Al, Ga)5O12:Ce.
However, a white color light-emitting device equipped with the blue color light-emitting element and the YAG-base phosphor emits white color light formed by the color mixture of blue light nearby 460 nm and yellow green light nearby 565 nm, but luminescence intensity nearby 500 nm is insufficient.
Further, there has been recently reported a white color light-emitting device combining a phosphor which emits blue light and a YAG-base phosphor which emits yellow light using light-emitting elements of visible light at a short wavelength side region. In this case, the YAG-base phosphor which emits yellow light is hardly excited by light of visible light at the short wavelength side region and does not emit light. Accordingly, a blue color-base phosphor is excited by said light-emitting element to emit blue light. Then, the YAG-base phosphor is excited by said blue light to emit yellow light. Thus, white color light is emitted by the color mixture of the blue light of the blue color-base phosphor with the yellow light of the YAG-base phosphor.
Various phosphors are developed as the phosphor used in said light-emitting device.
For example, an oxide-base phosphor using a rare earth metal element for a luminescence center has been widely known, and a portion of the phosphor is already practically used. However, a nitride phosphor and an oxynitride phosphor are seldom studied, and a study report is scarcely reported. For example, there is an oxynitride glass phosphor which is represented by Si—O—N, Mg—Si—O—N, Ca—Al—Si—O—N and the like (JP-A-2001-214162: hereinafter, referred to as the patent literature 1). Further, there is an oxynitride glass phosphor represented by Ca—Al—Si—O—N in which Eu was activated (JP-A-2002-76434: hereinafter, referred to as the patent literature 2).
However, conventional phosphors have low luminescence brightness and are insufficient for being used for a light-emitting device. In a light-emitting device using light-emitting elements at a near ultraviolet region as an excitation light source, there is used double step excitation that a blue light-base phosphor is excited by said light-emitting elements and the YAG-base phosphor is excited by said excited light, therefore while light having high efficiency is hardly obtained. Accordingly, there is desired a phosphor emitting green light to yellow light whose wavelength was directly converted by light of visible light at a short wavelength side region.
Further, a white color light-emitting device combining a phosphor and a light-emitting element of visible light at a short wavelength side region is not produced yet and the light-emitting device practically used is not commercially available. Accordingly, a phosphor which efficiently emits light at a short wavelength side region of visible light is desired.
Further, the above-mentioned oxynitride phosphors and the like described in the patent literatures 1 and 2 have low luminescence brightness and are insufficient for being used for the light-emitting device. Further, since the oxynitride glass phosphor is a glass body, it is hardly processed in general.